In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), organic photodetectors (OPDs), organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example of between 50 and 300 nm thickness.
One particular area of importance are OFETs. The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art, various materials have been proposed for use as organic semiconductors in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processability, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement
Another particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies above 8%.
In order to obtain ideal solution-processable OSC molecules two basic features are essential, firstly a rigid 71-conjugated core unit to form the backbone, and secondly a suitable functionality attached to the aromatic core unit in the OSC backbone. The former extends %-% overlaps, defines the primary energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), enables both charge injection and transport, and facilitates optical absorption. The latter further fine-tunes the energy levels and enables solubility and hence processability of the materials as well as π-π interactions of the molecular backbones in the solid state.
A high degree of molecular planarity reduces the energetic disorder of OSC backbones and accordingly enhances charge carrier mobilities. Linearly fusing aromatic rings is an efficient way of achieving maximum planarity with extended π-π conjugation of OSC molecules. Accordingly, most of the known polymeric OSCs with high charge carrier mobilities are generally composed of fused ring aromatic systems and are semicrystalline in their solid states. On the other hand, such fused aromatic ring systems are often difficult to synthesize, and do also often show poor solubility in organic solvents, which renders their processing as thin films for use in OE devices more difficult. Also, the OSC materials disclosed in prior art still leave room for further improvement regarding their electronic properties.
Thus there is still a need for organic semiconducting (OSC) materials for use in electronic devices like OFETs, which have advantageous properties, in particular good processability, especially a high solubility in organic solvents, good structural organization and film-forming properties, high charge-carrier mobility, high on/off ratio in transistor devices, high oxidative stability and long lifetime in electronic devices. In addition, the OSC materials should be easy to synthesize, especially by methods suitable for mass production. For use in OPV cells, the OSC materials should have a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to OSC materials of the prior art.
It was an aim of the present invention to provide OSC materials that provide one or more of the above-mentioned advantageous properties. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing conjugated polymers as disclosed and claimed hereinafter. These polymers comprise one or more thieno[3,2-b]thiophene based polycyclic units as represented by the formula I shown hereinafter, optionally together with further aromatic co-units. The thieno[3,2-b]thiophene based polycyclic units are tetrasubstituted at the cyclopentadiene rings.
Surprisingly it was found that these enlarged fused ring systems, and the polymers containing them, still show sufficient solubility in organic solvents by introduction of alkyl or alkylidene substituents. Both the homo- and co-polymers can be prepared through known transition metal catalysed polycondensation reactions. As a result the polymers of the present invention were found to be attractive candidates for solution processable organic semiconductors both for use in transistor applications and photovoltaic applications. By further variation of the substituents on the fused aromatic ring system, the solubility and electronic properties of the monomers and polymers can be further optimised.
Y.-J. Cheng et al., Chem. Commun., 2012, 48, 3203, disclose polymers with thieno[3,2-b]thiophene based polycyclic units (1) as shown below, wherein R is an alkyl group. However, a blend of such a polymer with PCBM was disclosed to have only very low mobilities of 1×10−4 cm2/Vs.

However, conjugated polymers as disclosed in the present invention and as claimed hereinafter have not been reported in prior art so far.